Liquefaction of Natural Gas

ABSTRACT

A method and apparatus for liquefying natural gas vapour is provided. Firstly, liquid natural gas is sub-cooled at a first heat exchanger using a liquid coolant such as liquid nitrogen. The sub-cooled liquid natural gas is then used to condense the natural gas vapour at a second heat exchanger.

FIELD OF THE INVENTION

The present invention relates to liquefaction of natural gas vapour,particularly during the transport and storage of liquefied natural gas(LNG). In particular, but not exclusively, the present invention relatesto the handling of boil off gas in a bunker vessel used to refuelLNG-powered vessels.

BACKGROUND TO THE INVENTION

Liquefied natural gas (LNG) is natural gas (typically methane—CH4) thathas been liquefied, typically to make it more manageable during storageand/or transport. At atmospheric pressure, this means the temperature ofthe LNG is reduced to around −163 degrees centigrade or below.

LNG carriers are vessels used to carry LNG across large distances,particularly to carry LNG from gas producing nations to gas consumingnations. The journey times of such vessels are significant, measured indays, weeks and even months. During such time, the LNG is stored at lowtemperature in order that it does not vaporise.

Notwithstanding the efforts made to maintain this low temperature, thereis in practice some evaporation of LNG during the vessel's journey. Thisevaporation creates boil-off gas (BOG) which must be handled by thevessel in some way. Indeed, this has been a factor in the continuing useof steam-turbine propulsion on LNG carriers. Carriers that use such apropulsion system are able to use BOG to drive turbines for propulsionand electricity generation.

However, steam turbine propulsion is a relatively inefficient means todrive a large ship. As a result, a move towards slow speed diesel enginepowered vessels has been apparent. The diesel engine in such a ship isunable to handle BOG and instead an independent LNG re-liquefactionplant is provided on the vessel. Typically nitrogen vapour is compressedand expanded in a “compander” in such a way as to lower its temperatureso that it can be used as a coolant during the re-liquefaction process.The re-liquefaction plant is typically powered by the electricitygenerated on-board and has significant requirements in this regard, withratings of 5 MW or above not uncommon. Furthermore, as well as highpower requirements, the re-liquefaction plant is typically unable tohandle high boil-off rates. Excess BOG that the LNG re-liquefactionplant is unable to handle is burnt in a gas combustion unit (GCU) andthereby wasted.

Another type of propulsion system used in LNG carriers is a dual fueldiesel electric propulsion system which can operate off either diesel orLNG itself In such a vessel, the BOG can be used as fuel for the enginesto provide propulsion and electrical load. If there is insufficientengine load it is necessary to burn the excess BOG in a GCU.

There has been an increasing drive towards the use of LNG as a primaryfuel in vessels of all types, driven at least partially by the relativecost of LNG in comparison to other fuels. As mentioned above, while thisoffers a potential method for handling BOG needed to provide propulsionand/or electricity, there remains the issue of handling such BOG whenthe required load does not match the BOG present.

Simply burning the BOG in a GCU is both wasteful and polluting. Nor canthe BOG simply be vented to the atmosphere, for similar reasons. Indeed,environmental regulations in many territories prohibit the handling ofBOG in this way within close proximity to shore. Furthermore, using LNGas a primary fuel source creates its own challenges. For example, inmany circumstances it may be desirable to re-fuel an LNG fuelled vesselusing a bunker vessel. A bunker vessel is a smaller vessel designed tocarry fuel to the carrier from shore. The bunker vessel must dock withthe carrier and transfer fuel from its tanks to that of the carrier.

As well as having to handle BOG during general operation, the bunkervessel must cope with the fact that the process of fuel transfer islikely to involve an increased production of BOG as the LNG istransferred between the vessels. Even if this were not the case the baselevel production of BOG would still need to be handled. However, as bothvessels are stationary during this process, the BOG created cannot beused for propulsion and must be handled in some other manner.

A re-liquefaction plant is impractical for a relatively small vesselsuch as a bunker vessel due to its large energy requirements and, asmentioned above, burning off excess BOG is both wasteful andenvironmentally unsound. There remains a need to find a method ofhandling BOG in bunker vessels that is both practical and effective.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod for condensing natural gas vapour to generate liquefied naturalgas (LNG), comprising:

providing a liquid coolant, wherein the liquid coolant has a boilingpoint less than that of natural gas;

cooling LNG at a first heat exchanger using the liquid coolant togenerate sub-cooled LNG; and

condensing natural gas vapour at a second heat exchanger using thesub-cooled LNG to liquefy the natural gas vapour and thereby generatefurther LNG.

According to a second aspect of the present invention, there is provideda system for condensing natural gas vapour to generate liquefied naturalgas (LNG); comprising

a first heat exchanger arranged to cool LNG using a liquid coolant togenerate sub-cooled LNG, wherein the liquid coolant has a boiling pointless than that of natural gas; and

a second heat exchanger arranged to condense natural gas vapour usingthe sub-cooled LNG to liquefy the natural gas vapour and therebygenerate further LNG.

The present invention can provide an efficient method of condensingnatural gas vapour to provide LNG. Existing LNG is cooled at a firstheat exchanger to create sub-cooled LNG that can be used to condense thenatural gas vapour. This is significantly more energy efficient thanprovision of a plant for generating LNG directly from the natural gasvapour. The use of a liquid coolant to first sub-cool existing availableLNG reduces the need for energy intensive refrigeration cycles togenerate a coolant to directly cool the natural gas vapour.

The present invention finds particular utility in circumstances in whichboil-off gas is present. Boil-off gas is natural gas vapour that hasarisen through the evaporation of an LNG source. In such circumstances,the boil-off gas is likely not to be significantly warmer than theboiling temperature of natural gas, and furthermore LNG is likely to beavailable for sub-cooling. Thus, in a preferred embodiment, the naturalgas vapour is boil-off gas.

The method may comprise receiving the LNG for cooling at the first heatexchanger from at least one LNG storage tank and returning thesub-cooled LNG to the at least one storage tank after it is used at thesecond heat exchanger. In this manner, LNG may be re-used for subsequentcondensing of natural gas vapour. After use at the second heat exchangerthe sub-cooled LNG is likely to have warmed somewhat, but the method ispreferably arranged such that it remains in liquid form for returning tothe LNG storage tank.

Preferably, the method may further comprise delivering the further LNGto the at least one storage tank. Accordingly, the LNG that is createdthrough the method may subsequently be used for condensing furthernatural gas vapour in future cycles. Furthermore, the system is compactand efficient in that there is no requirement for separate tanks for thecoolant used at the second heat exchanger and the generated LNG.

As mentioned above, the liquid coolant used at the first generator has aboiling point less than that of the natural gas vapour. The natural gasvapour is preferably predominantly methane, more preferably at least 90Mole % methane, at least 95 Mole % methane or over 97% Mole methane.Methane has a boiling point at atmospheric pressure of around −163degrees centigrade. Preferably, the liquid coolant has a boiling pointat atmospheric pressure of less than that of methane (i.e. less than−163 degrees centigrade), more preferably less than −170 degreescentigrade, and most preferably less than −190 degrees centigrade. Inpreferred embodiments, the liquid coolant is liquid nitrogen. Theboiling point of nitrogen is around −193 degrees centigrade atatmospheric pressure. References to atmospheric pressure refer to apressure equal to the understood unit of one atmosphere, rather than toprevailing climatic conditions.

The system may comprise a storage facility for the liquid coolant. Forexample, the system may comprise at least one storage tank for storingthe liquid coolant. The system may additionally or alternativelycomprise a generator for generating the liquid coolant. Equally, themethod may comprise a step of generating the liquid coolant. Forexample, where the system is provided on a maritime vessel, an initialsupply of liquid coolant may be provided to the storage facility from anon-shore facility while the vessel is docked, but when the vessel is atsea additional liquid coolant may be provided as needed by thegenerator.

According to a further aspect, a vessel may be provided comprising thesystem of the second aspect. The vessel may comprise a flow boomarranged to transfer LNG to a second vessel and to receive natural gasvapour from the second vessel. The method may further comprisetransferring LNG from a first vessel to a second vessel, and receivingthe natural gas vapour from the second vessel. In preferred embodiments,one or both vessels are maritime vessels. In particular, the vesselcomprising the system (i.e. the first vessel) may be a bunker vesselwhile the second vessel may be an LNG fuelled vessel. Alternatively oradditionally, the second vessel may be a carrier, particularly an LNGcarrier. A bunker vessel is a vessel arranged to re-fuel other vesselswhile at sea. Preferably, one or both vessels are LNG powered vessels.

BRIEF DESCRIPTION OF THE FIGURES

A preferred embodiment of the present invention will now be describedwith reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing a bunker vessel comprising asystem or handling LNG;

FIG. 2 is a schematic diagram showing the connection between variouselements of the system in more detail; and

FIG. 3 is a schematic diagram illustrating the components of are-condenser unit.

DETAILED DESCRIPTION

Referring to FIG. 1, a schematic diagram is provided showing theprinciple elements of a LNG handling system aboard a bunker vessel 1 forre-fuelling larger ships. The system comprises two LNG storage tanks 10,port and starboard manifolds 20, a liquid nitrogen tank 30, a gaseousnitrogen tank 40, a liquid nitrogen generator 50, a re-condenser unit60, engines 70 and a flow boom 80. FIG. 1 illustrates functionalconnections between the various illustrated elements of the system forthe transfer of LNG (indicated as “L”, dotted lines), natural gas vapour(indicated as “V”, thick lines) and nitrogen (both liquid and gaseousindicated as “N”, thin lines).

The port and starboard manifolds 20 are arranged to allow transfer ofLNG, natural gas vapour, and nitrogen between the bunker vessel and anon-shore facility. LNG received from the port and starboard manifolds isstored in the storage tanks 10. In this embodiment, the storage tanksare pressurised, C-class, storage tanks which may operate at up to 10bar, but the skilled person will recognize that alternative tanks may beused. Indeed, the storage tanks may be any type of chamber or containersuitable to act as a reservoir for LNG. Pressurised tanks, particularlypressurised “C” Type tanks, allow a wider range of operating temperatureand pressure within the bunker vessel. The storage tanks store LNG ataround −163 degrees centigrade.

Liquid nitrogen received from the manifolds 20 is stored in liquidnitrogen tank 30. Nitrogen that evaporates from liquid nitrogen tank 30may be received in gas nitrogen tank 40 from where it is passed to theship's systems for use in for purging of cargo/fuel lines, inertingtanks and hold spaces and so on. Alternatively or additionally, nitrogenmay be vented to the atmosphere or re-used by liquid nitrogen generator50. The liquid nitrogen tank 30 and the gas nitrogen tank 40 may be anysuitable container or chamber suitable to act as a reservoir for liquidnitrogen and gaseous nitrogen respectively.

The liquid nitrogen generator 50 passes generated liquid nitrogen backto the liquid nitrogen tank 30. The liquid nitrogen generator 50 may bea compression cooling system, and is preferably powered by the vessel'selectrical systems. For example, the liquid nitrogen generator 50 may bearranged to filter and compress atmospheric air before carbon dioxidewater and residual hydrocarbons are removed in an air purification unit.The air is then passed to a cold box where it is cooled and liquefied.The liquid air is distilled then in a distillation column to yield purenitrogen gas which is condensed in a condenser to yield pure liquidnitrogen.

The system comprises a flow boom 80 for transferring LNG to anothervessel, such as an LNG carrier. The flow boom 80 is a transfer boom inthis embodiment. The transfer boom 80 receives LNG from LNG storage tank10 and may also receive natural gas vapour from the other vessel.Natural gas vapour received by the transfer boom is passed to there-condenser unit 60.

The re-condenser unit 60 also receives natural gas vapour from the LNGstorage tank 10. The re-condenser unit 60 is arranged to re-liquefy thenatural gas vapour to return it to an LNG state. Once re-liquefied theLNG may be passed back to the LNG storage tank 10.

The re-condenser unit 60 also receives liquid nitrogen from the liquidnitrogen tank 30 and LNG from the LNG tanks 10. These liquids are usedduring the processes of cooling the natural gas vapour at there-condenser unit, as will be explained in greater detail below withreference to FIG. 3.

In the embodiment shown in FIG. 1, the engines 70 of the bunker vesseluse the stored LNG as a fuel source. In practice, natural gas vapour isused for combustion in the engines. The natural gas vapour used for thispurpose may be boil-off gas (BOG) spontaneously occurring in the LNGstorage tanks 10, or may be deliberately vaporised LNG from the LNGstorage tanks. LNG from the storage tank may be vaporised by a forcingvaporizer, for example.

FIG. 2 shows in more detail the connections between various elements ofthe system of FIG. 1. In particular, FIG. 2 illustrates the LNG storagetanks 10 and the port and starboard manifolds 20. As shown, a variety ofvalves are provided to control the flows and pressures of gas and liquidin the system. FIG. 2 also illustrates various connection points toother elements of the system. In particular, connection points 201 areprovided for LNG returning from the re-condenser unit 60, whileconnection points 202 are provided for natural gas vapour passed to there-condensing unit 60. There is also provided connection point 210 forpassing LNG to the re-condensing unit 60 for use during are-liquefaction process implemented by the re-condensing unit 60 and aconnection point 214 for receiving LNG from the re-condensing unit thathas been used for this purpose. LNG that is extracted from the LNGstorage tanks 10 but is not ultimately used in the engine 70 or there-condensing unit 60 may be returned to the LNG storage tanks viaconnection points 215.

Connection point 203 is shown for passing LNG to the transfer boom 80,while connection point 204 receives natural gas vapour from the transferboom 80. In this manner, LG can be transferred to another vessel, whileexcess boil-off as from the vessel can be retrieved for handling on thebunker vessel.

Connection point 205 is provided for passing gas vapour to the engines70. There is also provided a connection point 206 for transferringnatural gas vapour to a gas combustion unit (GCU). In an emergencysituation, this GCU may be used to burn and thus dispose of natural gasvapour that is not otherwise handled by the system. There is alsoprovided a vent 208 to vent natural gas vapour to the atmosphere wherethis is appropriate.

Connection point 207 is provided for the receipt of nitrogen vapour forpurging of lines, and inerting spaces as required. Excess vapour wouldbe vented via the GCU. The vapour may be received from the gas nitrogentank 40 which receives boil-off from the liquid nitrogen tank 30.Connection point 210 is for supply of liquid nitrogen via the manifolds20 from shore for charging the liquid nitrogen storage tank 30.

Each LNG storage tank is provided with a discharge pump 11 for pumpingLNG to the transfer boom 80 and a LNG fuel pump 12 for pumping LNG tothe engines 70. Various connections allow the LNG pumped by either thedischarge pump 11 or the LNG fuel pump 12 to be re-directed asappropriate.

Each LNG storage tank 10 also comprises a first LNG inlet 13 and asecond LNG inlet 14. LNG can be received at the LNG storage tanks 10from the re-condenser unit 60 via connection points 201 and from theport and starboard manifolds 20. The LNG storage tanks also comprise agas dome 15 above the storage tanks, where boil off gas (BOG) from thestored LNG is collected. The LNG storage tanks 10 comprise a gas outlet17 for passing this natural gas vapour to other elements of the system.

The LNG storage tanks 10 also comprise a return spray header 16. Thespray header returns a proportion of the LNG extracted from the LNGstorage tank 10 as a spray applied to the surface of the stored LNG inthe tank. This helps to maintain uniform temperature within the storedLNG and thereby reduces the rate of generation of boil off gas.

The port and starboard manifolds 20 comprise a first LNG interface 21. Asecond LNG interface 22, a natural gas vapour interface 23 and anitrogen interface 24. When the bunker vessel is docked, LNG may beprovided to the LNG storage tanks 10 via the LNG interfaces 21, 22 whilenatural gas vapour may be returned to shore via the gas vapour interface23. Liquid nitrogen may also be provided to the liquid nitrogen tank 30via the nitrogen interface 24.

FIG. 2 also illustrates a forcing vaporiser 211 on the line between theLNG storage tanks and connection point 205 to the engines 70. This isused to vaporise LNG to produce natural gas vapour which can becombusted by the engines 70 to generate power for the bunker vessel'spropulsion and electrical systems. The system further comprises acompressor 212 for compressing gas passed through the vaporiser 211before it reaches the engines 70. A further compressor 213 may also beprovided for natural gas vapour being returned to the port and starboardmanifolds 20.

FIG. 3 illustrates the re-condensing unit 60 in more detail. In order tofacilitate comparison with FIG. 2, various functionally equivalentconnections portions are shown in FIG. 3 using the same referencenumerals as used in FIG. 2. For example, FIG. 3 shows connection points202 for providing natural gas vapour, particularly BOG, to there-condensing unit 60. Moreover, connection points 201 for receiving there-liquefied LNG from the re-condensing unit 60 and returning this tothe LNG storage tanks 10 are also shown.

FIG. 3 also illustrates liquid nitrogen tank 30 and gaseous nitrogentank 40. The liquid nitrogen tank 30 is filled from the port andstarboard manifolds via connection point 209 (also shown in FIG. 2) andmay also be filled from bunker vessel's liquid nitrogen generator 50 viaconnection point 301. Nitrogen which evaporates from the liquid nitrogentank 30 may be passed to the gaseous nitrogen tank 40, from where it maybe vented via connection point 302 or passed to consumers via connectionpoint 303. Consumers in this case may include systems aboard the vessel;for example, nitrogen vapour may be used for purging of cargo/fuellines, inerting tanks and hold spaces and so on.

The re-condensing unit 60 comprises a first heat exchanger 62, a secondheat exchanger 64 and a compressor 66. The first heat exchanger 62 is anLNG sub-cooler and is coupled to the liquid nitrogen tank 30 and to LNGoutlets of the LNG tanks 10. LNG from the LNG tanks 10 is cooled usingliquid nitrogen from the liquid nitrogen tank 30 to below itstemperature in the LNG tanks 10. The LNG cooled in this manner isreferred to as “sub-cooled”.

The second heat exchanger 64 is coupled to the first heat exchanger 62so as to receive the sub-cooled LNG therefrom. The second heat exchanger64 is also arranged to receive natural gas vapour. The natural gasvapour may originate either at the LNG tanks 10 or be received fromanother vessel via the transfer boom 80. Typically, the natural gasvapour is BOG that has occurred by evaporation of LNG.

The second heat exchanger 64 is a condenser arranged to cool the naturalgas vapour using the sub-cooled LNG received from the first heatexchanger such that it is liquefied. The second heat exchanger thusgenerates LNG which is returned to the LNG tanks 10. Furthermore, onceit has passed through the second heat exchanger, the sub-cooled LNG isreturned to the LNG tanks 10.

The re-condensing unit also comprises a compressor 66. The compressor 66is used to compress natural gas vapour prior to its injection into thesecond heat exchanger 64. This is found to increase the efficiency ofheat exchange at the second heat exchanger 64.

In use, the bunker vessel comprising the system illustrated in FIG. 1 isdocked with another vessel which it is to re-fuel. In particularexamples, this other ship is an LNG fuelled vessel.

The transfer boom 80 is used to transfer LNG from the LNG tanks 10 tothe LNG carrier. During this process, natural gas vapour is displacedfrom the tanks aboard the LNG carrier, and boil-off gas is alsogenerated from the LNG tanks 10 on the bunker vessel and at other pointsin the system. This natural gas vapour is directed towards there-condensing unit, where it first encounters 60 the compressor 66. Thecompressor 66 acts to increase the pressure within the natural gasvapour by compressing the vapour, and the compressed vapour is thenpassed to the second heat exchanger 64. The second heat exchanger 64transfers heat between sub-cooled LNG and the compressed vapour, therebycooling the compressed vapour until it condenses (i.e. liquefies), andcreating LNG. This LNG is then passed to the LNG storage tanks 10.

As mentioned above, the sub-cooled LNG used in the second heat exchanger64 is also passed to the LNG storage tanks 10 after passing through thesecond heat exchanger 64. Prior to this, the sub-cooled LNG is generatedby cooling using liquid nitrogen at the first heat exchanger 62.

As described above, the preferred embodiment is designed for use upon abunker vessel used to refuel another ship. However, it will beunderstood that variations may be proposed for alternative types ofmarine vessels, and indeed to shore-based transport and storage.

Other variations and modifications will be apparent to the skilledperson. Such variations and modifications may involve equivalent andother features which are already known and which may be used instead of,or in addition to, features described herein. Features that aredescribed in the context of separate embodiments may be provided incombination in a single embodiment. Conversely, features which aredescribed in the context of a single embodiment may also be providedseparately or in any suitable sub-combination. It should be noted thatthe term “comprising” does not exclude other elements or steps, the term“a” or “an” does not exclude a plurality, a single feature may fulfillthe functions of several features recited in the claims and referencesigns in the claims shall not be construed as limiting the scope of theclaims. It should also be noted that the Figures are not necessarily toscale; emphasis instead generally being placed upon illustrating theprinciples of the present disclosure.

1. A method for condensing natural gas vapour to generate liquefied natural gas (LNG), comprising: providing a liquid coolant, wherein the liquid coolant has a boiling point less than that of natural gas; cooling LNG at a first heat exchanger using the liquid coolant to generate sub-cooled LNG; and condensing natural gas vapour at a second heat exchanger using the sub-cooled LNG to liquefy the natural gas vapour and thereby generate further LNG.
 2. A method according to claim 1, further comprising: obtaining the LNG for cooling at the first heat exchanger from at least one LNG storage tank; and returning the sub-cooled LNG to the at least one LNG storage tank after it is used at the second heat exchanger.
 3. A method according to claim 2, further comprising delivering the further LNG to the at least one LNG storage tank.
 4. A method according to claim 1, wherein the natural gas vapour is boil-off gas.
 5. A method according to claim 1, wherein the liquid coolant is liquid nitrogen.
 6. A method according to claim 1, further comprising compressing the natural gas vapour.
 7. A method according to claim 1, further comprising: delivering LNG from a first vessel to a second vessel; and receiving the natural gas vapour from the second vessel.
 8. A system for condensing natural gas vapour to generate liquefied natural gas (LNG); comprising: a first heat exchanger arranged to cool LNG using a liquid coolant to generate sub-cooled LNG, wherein the liquid coolant has a boiling point less than that of natural gas; and a second heat exchanger arranged to condense natural gas vapour using the sub-cooled LNG to liquefy the natural gas vapour and thereby generate further LNG.
 9. A system according to claim 8, further comprising at least one LNG storage tank, wherein the first heat exchanger is arranged to receive the LNG for cooling at the first heat exchanger from the at least one LNG storage tank, and wherein the system is arranged to return the sub-cooled LNG to the LNG storage tank after it is used at the second heat exchanger.
 10. A system according to claim 9, wherein the system is arranged to deliver the further LNG to the at least one LNG storage tank.
 11. A system according to claim 8, wherein the natural gas vapour is boil-off gas.
 12. A system according to claim 8, wherein the liquid coolant is liquid nitrogen.
 13. A system according to claim 8, further comprising a compressor for compressing the natural gas vapour.
 14. A vessel having a system for condensing natural gas vapour to generate liquefied natural gas, the vessel comprising: a first heat exchange exchanger arranged to cool LNG using a liquid coolant to generate sub-cooled LNG, wherein the liquid coolant has a boiling point less than that of natural gas; a second heat exchanger arranged to condense natural gas vapour using the sub-cooled LNG to liquefy the natural gas vapour and thereby generate further LNG; and a transfer boom arranged to transfer LNG to a second vessel and to receive natural gas vapour from the second vessel.
 15. A vessel according to claim 14, wherein the vessel is a bunker vessel. 